1. Field of the Invention
The present invention relates generally to a communication system that determines a modulation and coding scheme adaptively, and in particular, to a method and apparatus for determining an optimum modulation scheme for retransmission in a given environment.
2. Description of the Related Art
In a future mobile communication system (3GPP2 1xEV-DV: 3rd Generation Partnership Project 2 1xEvolution-Data and Voice, or 3GPP HSDPA: 3GPP High Speed Downlink Packet Access), if the number of available Walsh codes, that is, an available Walsh code space is given, the size of an encoder packet (EP) to be transmitted and the number of slots per sub-packet are determined according to channel condition and data backlog. A slot is a transmission unit having a predetermined time span and data backlog indicates the state of a buffer that stores data received from a higher layer. If the EP size and the number of slots per sub-packet are determined, this implies that a data rate is determined. An EP is then transmitted by selecting one of as many combinations of modulation schemes and coding rates as modulations supported in the system. For efficient packet transmission, it is very important to select an optimum modulation and coding scheme that minimizes BER (Bit Error Rate) and PER (Packet Error Rate) because the error rate of each modulation and coding scheme is different.
According to a proposed modulation scheme determining method for 3GPP2 1xEV-DV under standardization, a modulation scheme is selected referring to a look-up table listing data rates mapped to modulation schemes on a one-to-one basis for each EP size in a given Walsh code space. The look-up table, was made to offer an optimum modulation and coding scheme only at an initial transmission, which minimizes BER or PER under a given condition. However, even if a sub-packet is to be retransmitted due to failure of the previous transmission, the look-up table is still used. Without considering the previous sub-packet transmissions, therefore, an inappropriate modulation scheme may be selected, resulting in degradation of system performance.